Multilayered paper containers have been widely used as containers for a variety of fruit drinks and lactic acid bacteria beverages. Since the paper containers are made by heat sealing a preform sheet comprising a paper and a heat-sealing resin layer provided on the inner surface of the paper. For the purpose of increasing gas barrier properties and light-shielding properties thereof, the preform sheet for containers has in some cases a multilayered structure having an aluminum foil inner layer and a heat-sealing resin inner layer on the aluminum foil layer. Olefin polymers such as polyethylene have heretofore been used mainly as the heat-sealing resin as mentioned above. The olefin polymers have such excellent properties as good flexibility as well as good heat-sealing properties, and, therefore, pinholes are not formed when folded. The preparation of such paper containers include as a first step a step of preparing cylindrical members from a laminate (the preform sheet) comprising a paper layer and a heat sealing layer. The so-called flame sealing method has often been employed in the step from the standpoint of heat sealing properties and productivity thereof, wherein the resin of the heat sealing layer is melted by a gas flame, etc., and the layers are quickly pressed each other.
Such a heat-sealed layer will directly contact with the contents such as drinks and beverages. The olefin polymer used as the heat-sealed layer tends to absorb flavor or fragrance of the contents, and, as a result, the containers have a drawback that certain types of packaged materials change their taste and flavor.
The use of polyesters or ethylene/vinyl alcohol copolymers as the inner layer resin is considered to be useful for the improvement of such a drawback. When a laminate including such an inner resin layer is wrap-sealed, the seal is usually formed between an olefin polymer such as polyethylene with which the outer layer of the paper is usually coated, and a polyester or an ethylene/vinyl alcohol copolymer as the inner layer. Flame sealing usually gives sufficient seal strength to the sealed paper containers. When an olefin polymer, and a polyester or an ethylene/vinyl alcohol copolymer are heat sealed by the wrap sealing, however, flavor leakage from the sealed portion becomes unavoidable. As a result, the use of such paper containers for drinks and beverages includes problems.
For the purpose of solving such problems, it seems effective to adopt the so-called hemming method or skive-hemming method for the preparation of cylindrical members wherein an edge along one side of a preform sheet (a laminate) for paper containers is folded back, the folded edge is put upon the other edge, and both inner layer resins are heat sealed together. It is, however, found that those cylindrical members prepared by the above-mentioned methods wherein the inner layer resins are sealed together by the flame sealing have such a serious disadvantage that seals excellent in seal strength cannot be obtained although the seal is formed between the same kind of materials.
In view of the present situation as described above, the present inventors have carried out researches for a preparation process of cylindrical members for paper containers excellent in gas barrier properties and flavor barrier properties. As a result, it has been found that the cylindrical members having desired properties such as sealing properties excellent in seal strength can be manufactured with high productivity by using a specific type of the inner layer resin and a seal structure, and using a specific type of laser beam.